In the medical field, when a biomedical electrode is used on a patient, it is generally provided with a metallic plate attached to conductive wires which are, in turn, attached to a monitoring apparatus. To improve conductivity between the skin and the electrode, a composition, usually a paste, a gel, or a cream, is applied between the metallic plate and the skin. The use of such a composition is often messy and offensive to the patient. In addition, the composition must be removed from the skin after use, either by wiping or by the use of solvents, both of which are also offensive. Furthermore, the electrode is usually secured to the patient by tape having a conventional pressure sensitive adhesive (PSA) thereon. Removal of the tape causes discomfort to the patient, and the composition of the adhesive of the tape may induce an allergic reaction in the patient.
To circumvent these difficulties, electrically conductive hydrogel adhesives have been developed. These hydrogel adhesives have replaced pastes, gels, and creams because they advantageously provide both conductivity and clean, residue-free removal from the skin surface after use. In addition, depending on the particular application and electrode design and on the nature of the hydrogel adhesive, hydrogel adhesives can be used alone--without a conventional PSA to secure the electrode to the skin. This is a result of the hydrogel's own specific adhesive properties.
Hydrogel adhesives differ from conventional PSA's in several respects. PSA's generally consist of polyacrylate polymer, or a polyolefin polymer combined with a tackifier additive. Monomers used to prepare polyacrylate PSA's are long chain esters of acrylic acid such as octyl acrylate. These particular polymers are inherently tacky and adhesive in nature. Polyolefin-based PSA's are prepared from rubber-like polymers such as polybutylene. Because these polymers are not inherently tacky, a tackifier is blended into the polymer base. Terpenes are generally used as tackifiers in these compositions. Most significantly, a conventional PSA contains no solvent, is non-aqueous, and will not dry out. These polymers are inherently adhesive and do not depend upon a swollen, crosslinked, watery formulation to provide tackiness and peel adhesion. They cannot be made electrically conductive by addition of water and electrolyte and they tend to be less skin friendly than hydrogel adhesives. Examples of typical PSA's are the adhesives coated on "Scotch" brand cellophane tape, masking tape, medical adhesive tape, and "Band-Aid" brand adhesive-bandages.
Hydrogel skin adhesives have proven useful in a variety of applications involving biomedical electrodes. Examples of such applications include use with electrocardiographic (ECG) electrodes, electrosurgical grounding pads, defibrillation electrodes, transcutaneous nerve stimulation electrodes, and iontophoretic drug delivery electrodes. Non-electrode applications are also becoming increasingly important. Examples include use with transdermal drug delivery patches, as adhesives for ostomy devices, as wound dressings, and with medical tapes.
Hydrogel skin adhesive compositions are generally composed of a crosslinked water soluble polymer network swollen with water as a solvent component. Humectant materials are usually added as co-solvents to provide slow drying or non-drying characteristics to the hydrogel adhesive. Humectant materials are usually polyols such as glycerol, sorbitol, or propylene glycol, or low molecular weight polyethylene oxide diols such as PEG 400 or PEG 600. Other additives can also be added to hydrogel adhesives for specific purposes. Examples include electrolytes such as sodium chloride for electrical conductivity, preservatives such as methyl paraben to prevent microbiological degradation, buffering agents such as sodium dihydrogen phosphate for pH control, and water soluble polymers such as polyacrylamide for viscosity modification.
The crosslinked water soluble polymer network can be formed by polymerization of a large variety of water soluble monomers in the presence of difunctional crosslinking agents by free radical polymerization techniques initiated by thermal or photochemical methods. Crosslinking of previously formed water soluble polymers can also be effected by complexation with difunctional species, such as divalent metal cations or difunctional organic reactants, or by exposure to ionizing radiation such as gamma-rays or electron beams. Many naturally occurring polymers such as gelatin can be reversibly and non-covalently crosslinked by manipulating gelation temperatures.
Examples of water soluble monomers subject to free radical polymerization for use as a hydrogel are acrylic acid, methacrylic acid, acrylamide, methacrylamide, and vinyl pyrrolidone. Diacrylate esters of polyethylene oxides are typical free radical crosslinking agents used in these compositions. Examples of hydrogel formation by the crosslinking of water soluble polymers are the crosslinking of carboxymethylcellulose by reaction with aluminum ion, the crosslinking of prepolymer polyisocyanates by reaction with water and organic diols, and the crosslinking of polyvinylpyrrolidone by electron beam radiation. Examples of naturally occurring polymers which form gels by thermal gelation are gelatin and karaya gum. There are thus a large number of hydrogel adhesives of diverse types.
One of the critical properties determining the usefulness of a hydrogel adhesive is its peel adhesion strength. High peel strength allows the hydrogel adhesive to be used without an accompanying PSA in many applications. Hydrogel skin adhesives with high peel strength provide a more secure attachment of electrodes (or other devices) to the patient. Correspondingly, low peel strength provides a less secure attachment, allowing easier removal of the hydrogel adhesive from the patient. Depending on the particular application, varying hydrogel adhesive peel strengths are required.
Another useful property of hydrogel adhesives is repositionability. "Repositionability" is defined as the ability of the adhesive hydrogel to be removed from one area of the skin and to be attached to another area without loss of peel adhesion properties. Other properties of hydrogel adhesives include tack adhesion and creep compliance.
The properties of a given hydrogel adhesive depend in large measure upon the nature of the crosslinked water soluble polymer that comprises the hydrogel adhesive. Presently, the nature of the crosslinked water soluble polymer used in the hydrogel adhesive fixes the properties of that adhesive. In order to vary the peel strength or repositionability of the adhesive, for example, different water soluble polymers may be used. Development of hydrogel adhesive properties is thus an empirical process at present. A water soluble polymer system that allows broad variation of hydrogel adhesive properties without changing the type of polymer is desirable.